Transwomen have an estimated HIV prevalence of 42.4% as of 2013 and there are clear racial/ethnic disparities in HIV risk. HIV prevention among transwomen continues to lag behind research with other key populations. Data on the sexual partners and transmission dynamics in these partnerships may hold the key to effective HIV prevention with transwomen. We propose to conduct a systematic population-based mixed methods study to examine the sexual partnerships of transwomen and identify factors impacting their HIV risk. A barrier to this research is the marginalization of transwomen and their sexual partners creating theoretical and logistical challenges in obtaining inclusive, diverse samples. In this exploratory R21, we propose adaptations of respondent-driven sampling (RDS) to reach 100 transwomen and recruit up to 200 of their sexual partners, capitalizing on RDS' ability to penetrate different, interconnected social groups. We will also conduct a nested qualitative couples study to gather data on ways stigma impacts sexual decision making and risk in partnerships. This proposal offers the field a rare opportunity to determine the partnerships and circumstances defining HIV risk for this key population. The ultimate goal of this research is to determine evidence-based targets for a HIV risk reduction intervention for transwomen and their sexual partners. As such, this study is highly responsive to the National HIV/AIDS strategy need for HIV risk indicators specific to transwomen. This study also falls within the National Institute of Mental Health Division of AIDS Research priorities for advancing the evidence base for interventions beyond the individual level with a focus on high-risk vulnerable populations. This proposal will also maximize the NIH investment in the project by providing data to inform the development of a pilot HIV prevention intervention to reduce HIV risk among transwomen and their sexual partners.